The present invention relates to a novel sputter deposition method for electroluminescent phosphors used in thick dielectric electroluminescent (TDEL) displays. More specifically, the invention relates to a dual source sputter deposition method in which one of the targets comprises a metal and the other comprises the remaining elemental constituents of the phosphor.
Thick dielectric electroluminescent displays (TDEL) provide a great advance in flat panel display technology. TDEL displays comprise a basic structure of a substrate upon which an electrically conductive film is deposited. A thick film layer consisting of a ferroelectric material is then deposited on the thick film layer. A phosphor film is deposited on the thick film layer followed by an optically transparent but electrically conductive film to form the second electrode in the structure.
The thick film dielectric structure provides for superior resistance to dielectric breakdown as well as a reduced operating voltage as compared to other types of flat displays. The thick film dielectric structure when deposited on a ceramic substrate withstands higher processing temperatures than for example, TFEL devices, which are typically fabricated on glass substrates. This increased high temperature tolerance facilitates annealing of phosphor films at higher temperatures to improve their luminosity. However, even with this enhancement, it is desired to achieve even higher quality phosphor luminance and color coordinates as demonstrated by cathode ray tube (CRT) displays.
A high luminosity full colour electroluminescent display requires the use of red, green and blue sub-pixels. Optical filters are needed to achieve the required colour coordinates for each sub-pixel. Consequently, the thin film phosphor materials used for each sub-pixel must be patterned so that there is minimal attenuation of the emission spectrum for each colour of pixel by the optical filters. For relatively low-resolution displays, the required patterning can be achieved by depositing the phosphor materials through a shadow mask. For displays with high resolution, however, the shadow mask technique does not provide adequate accuracy, and photolithographic methods must be employed. Photolithographic techniques require the deposition of photoresist films and the etching or lift-off of portions of the phosphor film to provide the required pattern.
Deposition and removal of photoresist films and etching or lift-off of phosphor films typically require the use of solvent solutions that contain water or other protic solvents. Some phosphor materials, for example strontium sulphide are susceptible to hydrolysis, and water and protic solvents may degrade the properties of the phosphor materials.
The deficiencies in phosphor materials are most severe with the phosphors used for blue sub-pixels, and may be compensated for to some extent by increasing the area of the blue sub-pixels relative to the area of the red and green sub-pixels. However, such a design modification demands increased performance from the phosphor materials used for the red and green phosphor materials, and requires the use of higher display operating voltages. The higher operating voltages increase the power consumption of the display, decrease the reliability and increase the cost of driving the electronics of the display.
Vacuum deposition is a method that can be used to produce and deposit phosphor films from a single source pellet using sputtering or electron beam evaporation. However, the yielded films do not exhibit high luminosity. Improved luminance of barium thioaluminate phosphors has been achieved by using a hopping electron beam deposition technique to deposit films from two source pellets. In this method, the stoichiometry of the deposited film is controlled by controlling the relative dwell time of the electron beam impinging on each of the two source materials. While effective, this technique is not well adapted for commercial production of large area displays and is difficult to control to compensate for changes in the evaporation rates from the two sources as the deposition proceeds and the source pellets are depleted.
Sputtering is a well known technique for making a phosphor layer. Sputtering can involve the use of a single, dual or multiple source for the production of a phosphor layer. Sputtering processes are described in for example, U.S. Pat. Nos. 4,389,295, 4,508,610, 4,675,092, 5,003,221 and 6,254,740.
To improve the stoichiometry of certain phosphors such as thioaluminate phosphors, dual sources are used for the deposition requiring added controls over the relative deposition rates for the different sources. The required relative evaporation rates must be calibrated for each specific piece of deposition equipment and the requirement for multiple sources constrains the design of the deposition equipment, generally adding to the cost of the equipment. Further, evaporation methods are not well suited for the deposition of large area films such as a required for the fabrication of large electronic displays such as those for the wall television application.
While the aforementioned generally provides various methods for deposition of phosphor films, it would be advantageous to provide a method for depositing a phosphor film that can be etched and is also stable in the display operation. Furthermore, it would desirable that such a method also be economic and practical for the deposition of phosphor films over large areas.
The present invention relates to a novel process for depositing large area multi-element thin films for electroluminescent phosphors that provide a high luminosity and a suitable emission color. It also relates to the deposition of a phosphor film that can be etched in a patterning step and then further heat treated to form a stable phosphor film. It also relates to reducing the cost of equipment used to deposit the phosphor materials onto the dielectric layers.
The present invention utilizes two sputtering targets to deposit an alkaline earth thioaluminate phosphor film onto a suitable substrate in a sulfur containing atmosphere. Typically an activator species selected to provide the desired light emission color is also added to one of the targets. One of the sputtering targets is elemental metal. The other sputtering target comprises the remaining ingredients in the phosphor so that the resultant phosphor composition comprises a thioaluminate composition in combination with one or more elements from group IIA and IIB of the Periodic Table of Elements. The activator species is added to one of the targets and is selected from the group consisting of alkaline earth sulfide and a rare earth sulfide or oxide and mixtures thereof.
The use of elemental metal as a target in the sputtering deposition process is an improvement over prior art sputtering processes. The use of elemental metal ensures a high sputter deposition rate and also acts to minimize oxygen contamination of the second target and deposited phosphor composition. As a result, the deposited phosphor composition is etchable, provides desirable luminescence and is stable in display operation. Elemental metals suitable for use in the present invention are those of Group IIIA of the Periodic Table of Elements. More preferably, the metals are selected from the group consisting of aluminum, gallium and indium Most preferred for use in the present invention is aluminum.
The second target is typically a sulfur containing compound. More preferably, the second target is a sulfide compound comprising one or more elements of Group IIA and/or Group IIB of the Periodic Table of Elements.
The activator species is preferably cerium or europium.
In accordance with the present invention, there is provided a dual source sputtering deposition method for deposition of a thin film phosphor composition which uses an elemental metal, preferably aluminum, as one of the two sources.
In accordance with a further aspect of the present invention, the method is conducted in a sulfur containing atmosphere. The sulfur containing atmosphere is preferably one of hydrogen sulfide or sulfur vapour under low pressure to provide sufficient sulfur in the deposited phosphor composition. The sulfur may be provided at pressures ranging from about 1 to 10xc3x9710xe2x88x923 torr, more preferably, 3 to 7xc3x9710xe2x88x923 torr, and even more preferably 4 to 6xc3x9710xe2x88x923 torr.
In accordance with a further aspect of the present invention, sputtering is done at a rapid sputtering rate to minimize oxygen incorporation into the deposited phosphor composition. This is facilitated by the use of aluminum as the preferred elemental metal as one of the two sources.
In accordance with another aspect of the present invention, the sputtering method of the present invention is suitable for the production of a thin film phosphor composition onto a desired and suitable substrate having a large surface area. This adds to the economic feasibility of the method and its large scale commercial use.
In accordance with still a further aspect of the present invention, the sputtering method of the present invention can be conducted modulating the relative deposition rate of materials arising from each of the two sources. This facilitates deposition of a laminated film with a periodic composition alternatively rich and poor in aluminum.
In accordance with yet another aspect of the present invention, the sputtering method of the present invention may be conducted such that sputtering of the two targets is effected onto a rotating and/or oscillating substrate to effect the deposition of a laminated phosphor composition. The rotating and/or oscillating substrate is alternately positioned in the flux of atomic species sputtered from each of the two targets. The thickness of the layers of the laminated composition may be altered by changing the rotation rate or the oscillation rate of the substrate.
In accordance with an aspect of the present invention is a dual source sputtering method for the deposition of a thin film phosphor composition onto a substrate, said method comprising the steps of:
providing a metal as a first source
providing a sulfur bearing compound as a second source;
doping said first or second source with a rare earth activator; and
applying sufficient current to the first and second sources in a sulfur containing atmosphere to effect sputtering of said target and deposition of said phosphor composition on said substrate.
In accordance with another aspect of the present invention is a dual source sputtering method for the deposition of a thin film phosphor composition onto a substrate, said method comprising the steps of:
placing a metal as a first source and a sulfur bearing compound as a second source within a chamber having an atmosphere of hydrogen sulfide or sulfur vapours;
doping said first or second source with a rare earth activator; and
applying sufficient electrical energy to said first and second target to cause sputtering thereof and a flux of atomic species of said first and second targets onto said substrate.
In accordance with a further aspect of the present invention is a thin film laminated phosphor comprising;
alternate layers of metal sulfide and rare earth doped alkaline earth sulfide.
In accordance with a further aspect of the present invention is a thin film laminated phosphor comprising;
alternate layers of aluminum sulfide and rare earth doped alkaline earth sulfide.
The methods of the present invention are particularly suitable for deposition of thioaluminate based blue phosphors.
In accordance with yet a further aspect of the present invention is a thick dielectric electroluminescent display comprising;
a rigid substrate;
a lower electrode layer directly adjacent said substrate, said lower electrode layer comprising an electrically conductive metallic film;
a thick film dielectric layer provided on an upper surface of said electrode layer;
a phosphor film deposited on said thick film dielectric layer, said phosphor film comprising an alkaline earth thioaluminate phosphor film; and
an upper electrode layer comprising an optically transparent electrically conductive film.
In accordance with yet a further aspect of the invention is a method for preparing a thin film phosphor composition, said method comprising the steps of;
placing a substrate with a reactive chamber;
supplying a sputtering gas mixture of hydrogen sulfide at a pressure of about 5xc3x9710xe2x88x923 torr to the reactive chamber;
applying power to a first source of elemental aluminum;
and a second source of alkaline earth sulfide in a ratio of about 1:1 to 5:1;
said first or second source additionally comprising an activator species of a rare earth sulfide or rare earth oxide;
wherein applying said power to said first and second sources causes sputtering thereof and a flux of atomic species of said first and second targets onto said substrate to form a thin film alkaline earth thioaluminate phosphor composition.
In accordance with yet a further aspect of the invention is a method for preparing a thin film phosphor composition, said method comprising the steps of;
placing a substrate with a reactive chamber;
supplying a sputtering gas mixture of hydrogen sulfide at a pressure of about 5xc3x9710xe2x88x923 torr to the reactive chamber;
applying power to a first source of elemental aluminum and
to a second source of alkaline earth sulfide in a ratio of about 1:1 to 5:1
said first or second source additionally comprising an activator species of a rare earth sulfide or rare earth oxide;
wherein applying varying levels of power to said first and second sources causes sputtering thereof and a flux of atomic species of said first and second targets onto said substrate to form a thin film laminated alkaline earth thioaluminate phosphor composition.